Focus scanning apparatus

ABSTRACT

A scanner includes a camera, a light source for generating a probe light incorporating a spatial pattern, an optical system for transmitting the probe light towards the object and for transmitting at least a part of the light returned from the object to the camera, a focus element within the optical system for varying a position of a focus plane of the spatial pattern on the object, unit for obtaining at least one image from said array of sensor elements, unit for evaluating a correlation measure at each focus plane position between at least one image pixel and a weight function, a processor for determining the in-focus position(s) of each of a plurality of image pixels for a range of focus plane positions, or each of a plurality of groups of image pixels for a range of focus plane positions, and transforming in-focus data into 3D real world coordinates.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 14/502,230,filed on Sep. 30, 2014, which is a continuation of U.S. Ser. No.13/376,427, filed on Dec. 6, 2011, now U.S. Pat. No. 8,878,905 B2, whichis a national stage application of PCT/DK2010/050148, filed on Jun. 17,2010, and which claims the benefit of U.S. Ser. No. 61/187,744, filed onJun. 17, 2009 and U.S. Ser. No. 61/231,118, filed on Aug. 4, 2009. Thecontents of U.S. Ser. No. 14/502,230; U.S. Ser. No. 13/376,427;PCT/DK2010/050148; U.S. 61/187,744; and U.S. Ser. No. 61/231,118 areincorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure relates to an apparatus and a method for optical3D scanning of surfaces. The principle of the apparatus and methodaccording to the disclosure may be applied in various contexts. Onespecific embodiment of the invention is particularly suited forintraoral scanning, i.e. direct scanning of teeth and surroundingsoft-tissue in the oral cavity. Other dental related embodiments of theinvention are suited for scanning dental impressions, gypsum models, waxbites, dental prosthetics and abutments. Another embodiment of theinvention is suited for scanning of the interior and exterior part of ahuman ear or ear channel impressions. The disclosure may find use withinscanning of the 3D structure of skin in dermatological orcosmetic/cosmetological applications, scanning of jewelry or wax modelsof whole jewelry or part of jewelry, scanning of industrial parts andeven time resolved 3D scanning, such as time resolved 3D scanning ofmoving industrial parts.

BACKGROUND

The disclosure relates to three dimensional (3D) scanning of the surfacegeometry of objects. Scanning an object surface in 3 dimensions is awell known field of study and the methods for scanning can be dividedinto contact and non-contact methods. An example of contact measurementsmethods are Coordinate Measurement Machines (CMM), which measures byletting a tactile probe trace the surface. The advantages include greatprecision, but the process is slow and a CMM is large and expensive.Non-contact measurement methods include x-ray and optical probes.

Confocal microscopy is an optical imaging technique used to increasemicrograph contrast and/or to reconstruct three-dimensional images byusing a spatial pinhole to eliminate out-of-focus light or flare inspecimens that are thicker than the focal plane.

A confocal microscope uses point illumination and a pinhole in anoptically conjugate plane in front of the detector to eliminateout-of-focus information. Only the light within the focal plane can bedetected. As only one point is illuminated at a time in confocalmicroscopy, 2D imaging requires raster scanning and 3D imaging requiresraster scanning in a range of focus planes.

In WO 00/08415 the principle of confocal microscopy is applied byilluminating the surface with a plurality of illuminated spots. Byvarying the focal plane in-focus spot-specific positions of the surfacecan be determined. However, determination of the surface structure islimited to the parts of the surface that are illuminated by a spot.

WO 2003/060587 relates to optically sectioning of a specimen inmicroscopy wherein the specimen is illuminated with an illuminationpattern. Focus positions of the image plane are determined bycharacterizing an oscillatory component of the pattern. However, thefocal plane can only be adjusted by moving the specimen and the opticalsystem relative to each other, i.e. closer to or further away from eachother. Thus, controlled variation of the focal plane requires acontrolled spatial relation between the specimen and the optical system,which is fulfilled in a microscope. However, such a controlled spatialrelation is not applicable to e.g. a hand held scanner.

US2007/0109559 A1 describes a focus scanner where distances are foundfrom the focus lens positions at which maximum reflective intensity oflight beams incident on the object being scanned is observed. Incontrast to the disclosure disclosed here, this prior art exploits nopre-determined measure of the illumination pattern and exploits nocontrast detection, and therefore, the signal-to-noise ratio issub-optimal.

In WO 2008/125605, means for generating a time-variant pattern composedof alternating split images are described. This document describes ascanning method to obtain an optical section of a scan object by meansof two different illumination profiles, e.g. two patterns of oppositephases. These two images are used to extract the optical section, andthe method is limited to acquisition of images from only two differentillumination profiles. Furthermore, the method relies on a predeterminedcalibration that determines the phase offset between the twoillumination profiles.

SUMMARY

Thus, an object of the disclosure is to provide a scanner which may beintegrated in a manageable housing, such as a handheld housing. Furtherobjects of the disclosure are: discriminate out-of-focus information andprovide a fast scanning time.

This is achieved by a method and a scanner for obtaining and/ormeasuring the 3D geometry of at least a part of the surface of anobject, said scanner comprising:

-   -   at least one camera accommodating an array of sensor elements,    -   means for generating a probe light incorporating a spatial        pattern,    -   means for transmitting the probe light towards the object        thereby illuminating at least a part of the object with said        pattern in one or more configurations,    -   means for transmitting at least a part of the light returned        from the object to the camera,    -   means for varying the position of the focus plane of the pattern        on the object while maintaining a fixed spatial relation of the        scanner and the object,    -   means for obtaining at least one image from said array of sensor        elements,    -   means for evaluating a correlation measure at each focus plane        position between at least one image pixel and a weight function,        where the weight function is determined based on information of        the configuration of the spatial pattern;    -   data processing means for:        -   a) determining by analysis of the correlation measure the            in-focus position(s) of:            -   each of a plurality of image pixels for a range of focus                plane positions, or            -   each of a plurality of groups of image pixels for a                range of focus plane positions, and        -   b) transforming in-focus data into 3D real world            coordinates.

The method and apparatus described in this disclosure is for providing a3D surface registration of objects using light as a non-contact probingagent. The light is provided in the form of an illumination pattern toprovide a light oscillation on the object. The variation/oscillation inthe pattern may be spatial, e.g. a static checkerboard pattern, and/orit may be time varying, for example by moving a pattern across theobject being scanned. The disclosure provides for a variation of thefocus plane of the pattern over a range of focus plane positions whilemaintaining a fixed spatial relation of the scanner and the object. Itdoes not mean that the scan must be provided with a fixed spatialrelation of the scanner and the object, but merely that the focus planecan be varied (scanned) with a fixed spatial relation of the scanner andthe object. This provides for a hand held scanner solution based on thepresent disclosure.

In some embodiments the signals from the array of sensor elements arelight intensity.

One embodiment of the invention comprises a first optical system, suchas an arrangement of lenses, for transmitting the probe light towardsthe object and a second optical system for imaging light returned fromthe object to the camera. In the preferred embodiment of the inventiononly one optical system images the pattern onto the object and imagesthe object, or at least a part of the object, onto the camera,preferably along the same optical axis, however along opposite opticalpaths.

In the preferred embodiment of the invention an optical system providesan imaging of the pattern onto the object being probed and from theobject being probed to the camera. Preferably, the focus plane isadjusted in such a way that the image of the pattern on the probedobject is shifted along the optical axis, preferably in equal steps fromone end of the scanning region to the other. The probe lightincorporating the pattern provides a pattern of light and darkness onthe object. Specifically, when the pattern is varied in time for a fixedfocus plane then the in-focus regions on the object will display anoscillating pattern of light and darkness. The out-of-focus regions willdisplay smaller or no contrast in the light oscillations.

Generally we consider the case where the light incident on the object isreflected diffusively and/or specularly from the object's surface. Butit is understood that the scanning apparatus and method are not limitedto this situation. They are also applicable to e.g. the situation wherethe incident light penetrates the surface and is reflected and/orscattered and/or gives rise to fluorescence and/or phosphorescence inthe object. Inner surfaces in a sufficiently translucent object may alsobe illuminated by the illumination pattern and be imaged onto thecamera. In this case a volumetric scanning is possible. Some plankticorganisms are examples of such objects.

When a time varying pattern is applied a single sub-scan can be obtainedby collecting a number of 2D images at different positions of the focusplane and at different instances of the pattern. As the focus planecoincides with the scan surface at a single pixel position, the patternwill be projected onto the surface point in-focus and with highcontrast, thereby giving rise to a large variation, or amplitude, of thepixel value over time. For each pixel it is thus possible to identifyindividual settings of the focusing plane for which each pixel will bein focus. By using knowledge of the optical system used, it is possibleto transform the contrast information vs. position of the focus planeinto 3D surface information, on an individual pixel basis.

Thus, in one embodiment of the disclosure the focus position iscalculated by determining the light oscillation amplitude for each of aplurality of sensor elements for a range of focus planes.

For a static pattern a single sub-scan can be obtained by collecting anumber of 2D images at different positions of the focus plane. As thefocus plane coincides with the scan surface, the pattern will beprojected onto the surface point in-focus and with high contrast. Thehigh contrast gives rise to a large spatial variation of the staticpattern on the surface of the object, thereby providing a largevariation, or amplitude, of the pixel values over a group of adjacentpixels. For each group of pixels it is thus possible to identifyindividual settings of the focusing plane for which each group of pixelswill be in focus. By using knowledge of the optical system used, it ispossible to transform the contrast information vs. position of the focusplane into 3D surface information, on an individual pixel group basis.

Thus, in one embodiment of the invention the focus position iscalculated by determining the light oscillation amplitude for each of aplurality of groups of the sensor elements for a range of focus planes.

The 2D to 3D conversion of the image data can be performed in a numberof ways known in the art. I.e. the 3D surface structure of the probedobject can be determined by finding the plane corresponding to themaximum light oscillation amplitude for each sensor element, or for eachgroup of sensor elements, in the camera's sensor array when recordingthe light amplitude for a range of different focus planes. Preferably,the focus plane is adjusted in equal steps from one end of the scanningregion to the other. Preferably the focus plane can be moved in a rangelarge enough to at least coincide with the surface of the object beingscanned.

The present disclosure distinguishes itself from WO 2008/125605, becausein the embodiments of the present invention that use a time-variantpattern, input images are not limited to two illumination profiles andcan be obtained from any illumination profile of the pattern. This isbecause the orientation of the reference image does not rely entirely ona predetermined calibration, but rather on the specific time of theinput image acquisition.

Thus WO 2008/125605 applies specifically exactly two patterns, which arerealized physically by a chrome-on-glass mask as illuminated from eitherside, the reverse side being reflective. WO 2008/125605 thus has theadvantage of using no moving parts, but the disadvantage of acomparatively poorer signal-to-noise ratio. In the present disclosurethere is the possibility of using any number of pattern configurations,which makes computation of the light oscillation amplitude or thecorrelation measure more precise.

Definitions

Pattern: A light signal comprising an embedded spatial structure in thelateral plane. May also be termed “illumination pattern”.

Time varying pattern: A pattern that varies in time, i.e. the embeddedspatial structure varies in time. May also be termed “time varyingillumination pattern”. In the following also termed “fringes”.

Static pattern: A pattern that does not vary in time, e.g. a staticcheckerboard pattern or a static line pattern.

Pattern configuration: The state of the pattern. Knowledge of thepattern configuration at a certain time amounts to knowing the spatialstructure of the illumination at that time. For a periodic pattern thepattern configuration will include information of the pattern phase. Ifa surface element of the object being scanned is imaged onto the camerathen knowledge of the pattern configuration amounts to knowledge of whatpart of the pattern is illuminating the surface element.

Focus plane: A surface where light rays emitted from the patternconverge to form an image on the object being scanned. The focus planedoes not need to be flat. It may be a curved surface.

Optical system: An arrangement of optical components, e.g. lenses, thattransmit, collimate and/or images light, e.g. transmitting probe lighttowards the object, imaging the pattern on and/or in the object, andimaging the object, or at least a part of the object, on the camera.

Optical axis: An axis defined by the propagation of a light beam. Anoptical axis is preferably a straight line. In the preferred embodimentof the invention the optical axis is defined by the configuration of aplurality of optical components, e.g. the configuration of lenses in theoptical system. There may be more than one optical axis, if for exampleone optical system transmits probe light to the object and anotheroptical system images the object on the camera. But preferably theoptical axis is defined by the propagation of the light in the opticalsystem transmitting the pattern onto the object and imaging the objectonto the camera. The optical axis will often coincide with thelongitudinal axis of the scanner.

Optical path: The path defined by the propagation of the light from thelight source to the camera. Thus, a part of the optical path preferablycoincides with the optical axis. Whereas the optical axis is preferablya straight line, the optical path may be a non-straight line, forexample when the light is reflected, scattered, bent, divided and/or thelike provided e.g. by means of beam splitters, mirrors, optical fibersand the like.

Telecentric system: An optical system that provides imaging in such away that the chief rays are parallel to the optical axis of said opticalsystem. In a telecentric system out-of-focus points have substantiallysame magnification as in-focus points. This may provide an advantage inthe data processing. A perfectly telecentric optical system is difficultto achieve, however an optical system which is substantially telecentricor near telecentric may be provided by careful optical design. Thus,when referring to a telecentric optical system it is to be understoodthat it may be only near telecentric.

Scan length: A lateral dimension of the field of view. If the probe tip(i.e. scan head) comprises folding optics to direct the probe light in adirection different such as perpendicular to the optical axis then thescan length is the lateral dimension parallel to the optical axis.

Scan object: The object to be scanned and on which surface the scannerprovides information. “The scan object” may just be termed “the object”.

Camera: Imaging sensor comprising a plurality of sensors that respond tolight input onto the imaging sensor. The sensors are preferably orderedin a 2D array in rows and columns.

Input signal: Light input signal or sensor input signal from the sensorsin the camera. This can be integrated intensity of light incident on thesensor during the exposure time or integration of the sensor. Ingeneral, it translates to a pixel value within an image. May also betermed “sensor signal”.

Reference signal: A signal derived from the pattern. A reference signalmay also be denoted a weight function or weight vector or referencevector.

Correlation measure: A measure of the degree of correlation between areference and input signal. Preferably the correlation measure isdefined such that if the reference and input signal are linearly relatedto each other then the correlation measure obtains a larger magnitudethan if they are not. In some cases the correlation measure is a lightoscillation amplitude.

Image: An image can be viewed as a 2D array of values (when obtainedwith a digital camera) or in optics, an image indicates that thereexists a relation between an imaged surface and an image surface wherelight rays emerging from one point on said imaged surface substantiallyconverge on one point on said image surface.

Intensity: In optics, intensity is a measure of light power per unitarea. In image recording with a camera comprising a plurality ofindividual sensing elements, intensity may be used to term the recordedlight signal on the individual sensing elements. In this case intensityreflects a time integration of light power per unit area on the sensingelement over the exposure time involved in the image recording.

Mathematical Notation

-   -   A A correlation measure between the weight function and the        recorded light signal. This can be a light oscillation        amplitude.    -   I Light input signal or sensor input signal. This can be        integrated intensity of light incident on the sensor during the        exposure time or integration of the sensor. In general, it        translates to a pixel value within an image.    -   f Reference signal. May also be called weight value.    -   n The number of measurements with a camera sensor and/or several        camera sensors that are used to compute a correlation measure.    -   H Image height in number of pixels    -   W Image width in number of pixels

Symbols are also explained as needed in the text.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The scanner preferably comprises at least one beam splitter located inthe optical path. For example, an image of the object may be formed inthe camera by means of a beam splitter. Exemplary uses of beam splittersare illustrated in the figures.

In a preferred embodiment of the invention light is transmitted in anoptical system comprising a lens system. This lens system may transmitthe pattern towards the object and images light reflected from theobject to the camera.

In a telecentric optical system, out-of-focus points have the samemagnification as in-focus points. Telecentric projection can thereforesignificantly ease the data mapping of acquired 2D images to 3D images.Thus, in a preferred embodiment of the invention the optical system issubstantially telecentric in the space of the probed object. The opticalsystem may also be telecentric in the space of the pattern and camera.

Varying Focus

A pivotal point of the disclosure is the variation, i.e. scanning, ofthe focal plane without moving the scanner in relation to the objectbeing scanned. Preferably the focal plane may be varied, such ascontinuously varied in a periodic fashion, while the pattern generationmeans, the camera, the optical system and the object being scanned isfixed in relation to each other. Further, the 3D surface acquisitiontime should be small enough to reduce the impact of relative movementbetween probe and teeth, e.g. reduce effect of shaking. In the preferredembodiment of the invention the focus plane is varied by means of atleast one focus element. Preferably the focus plane is periodicallyvaried with a predefined frequency. Said frequency may be at least 1 Hz,such as at least 2 Hz, 3, 4, 5, 6, 7, 8, 9 or at least 10 Hz, such as atleast 20, 40, 60, 80 or at least 100 Hz.

Preferably the focus element is part of the optical system. I.e. thefocus element may be a lens in a lens system. A preferred embodimentcomprises means, such as a translation stage, for adjusting andcontrolling the position of the focus element. In that way the focusplane may be varied, for example by translating the focus element backand forth along the optical axis.

If a focus element is translated back and forth with a frequency ofseveral Hz this may lead to instability of the scanner. A preferredembodiment of the invention thus comprises means for reducing and/oreliminating the vibration and/or shaking from the focus elementadjustment system, thereby increasing the stability of the scanner. Thismay at least partly be provided by means for fixing and/or maintainingthe centre of mass of the focus element adjustment system, such as acounter-weight to substantially counter-balance movement of the focuselement; for example, by translating a counter-weight opposite to themovement of the focus element. Ease of operation may be achieved if thecounter-weight and the focus element are connected and driven by thesame translation means. This may however, only substantially reduce thevibration to the first order. If a counter-weight balanced device isrotated around the counter-weight balanced axis, there may be issuesrelating to the torque created by the counter-weights. A furtherembodiment of the invention thus comprises means for reducing and/oreliminating the first order, second order, third order and/or higherorder vibration and/or shaking from the focus element adjustment system,thereby increasing the stability of the scanner.

In another embodiment of the invention more than one optical element ismoved to shift the focal plane. In that embodiment it is desirable thatthese elements are moved together and that the elements are physicallyadjacent.

In the preferred embodiment of the invention the optical system istelecentric, or near telecentric, for all focus plane positions. Thus,even though one or more lenses in the optical system may be shifted backand forth to change the focus plane position, the telecentricity of theoptical system is maintained.

The preferred embodiment of the invention comprises focus gearing. Focusgearing is the correlation between movement of the lens and movement ofthe focus plane position. E.g. a focus gearing of 2 means that atranslation of the focus element of 1 mm corresponds to a translation ofthe focus plane position of 2 mm. Focus gearing can be provided by asuitable design of the optical system. The advantage of focus gearing isthat a small movement of the focus element may correspond to a largevariation of the focus plane position. In specific embodiments of theinvention the focus gearing is between 0.1 and 100, such as between 0.1and 1, such as between 1 and 10, such as between 2 and 8, such asbetween 3 and 6, such as least 10, such as at least 20.

In another embodiment of the invention the focus element is a liquidlens. A liquid lens can control the focus plane without use of anymoving parts.

Camera

The camera may be a standard digital camera accommodating a standard CCDor CMOS chip with one A/D converter per line of sensor elements(pixels). However, to increase the frame rate the scanner according tothe disclosure may comprise a high-speed camera accommodating multipleA/D converters per line of pixels, e.g. at least 2, 4, 8 or 16 A/Dconverters per line of pixels.

Pattern

Another central element of the disclosure is the probe light with anembedded pattern that is projected on to the object being scanned. Thepattern may be static or time varying. The time varying pattern mayprovide a variation of light and darkness on and/or in the object.Specifically, when the pattern is varied in time for a fixed focus planethen the in-focus regions on the object will display an oscillatingpattern of light and darkness. The out-of-focus regions will displaysmaller or no contrast in the light oscillations. The static pattern mayprovide a spatial variation of light and darkness on and/or in theobject. Specifically, the in-focus regions will display an oscillatingpattern of light and darkness in space. The out-of-focus regions willdisplay smaller or no contrast in the spatial light oscillations.

Light may be provided from an external light source, however preferablythe scanner comprises at least one light source and pattern generationmeans to produce the pattern. It is advantageous in terms ofsignal-to-noise ratio to design a light source such that the intensityin the non-masked parts of the pattern is as close to uniform in spaceas possible. In another embodiment the light source and the patterngeneration means is integrated in a single component, such as asegmented LED. A segmented LED may provide a static pattern and/or itmay provide a time varying pattern in itself by turning on and off thedifferent segments in sequence. In one embodiment of the invention thetime varying pattern is periodically varying in time. In anotherembodiment of the invention the static pattern is periodically varyingin space.

Light from the light source (external or internal) may be transmittedthrough the pattern generation means thereby generating the pattern. Forexample the pattern generation means comprises at least one translucentand/or transparent pattern element. For generating a time varyingpattern a wheel, with an opaque mask can be used. E.g. the maskcomprises a plurality of radial spokes, preferably arranged in asymmetrical order. The scanner may also comprise means for rotatingand/or translating the pattern element. For generating a static patterna glass plate with an opaque mask can be used. E.g. the mask comprises aline pattern or checkerboard pattern. In general said mask preferablypossesses rotational and/or translational periodicity. The patternelement is located in the optical path. Thus, light from the lightsource may be transmitted through the pattern element, e.g. transmittedtransversely through the pattern element. The time varying pattern canthen be generated by rotating and/or translating the pattern element. Apattern element generating a static pattern does not need to be movedduring a scan.

Correlation

One object of the disclosure is to provide short scan time and real timeprocessing, e.g. to provide live feedback to a scanner operator to makea fast scan of an entire tooth arch. However, real time high resolution3D scanning creates an enormous amount of data. Therefore dataprocessing should be provided in the scanner housing, i.e. close to theoptical components, to reduce data transfer rate to e.g. a cart,workstation or display. In order to speed up data processing time and inorder to extract in-focus information with an optimal signal-to-noiseratio various correlation techniques may be embedded/implemented. Thismay for example be implemented in the camera electronics to discriminateout-of-focus information. The pattern is applied to provide illuminationwith an embedded spatial structure on the object being scanned.Determining in-focus information relates to calculating a correlationmeasure of this spatially structured light signal (which we term inputsignal) with the variation of the pattern itself (which we termreference signal). In general the magnitude of the correlation measureis high if the input signal coincides with the reference signal. If theinput signal displays little or no variation then the magnitude of thecorrelation measure is low. If the input signal displays a large spatialvariation but this variation is different than the variation in thereference signal then the magnitude of the correlation measure is alsolow. In a further embodiment of the invention the scanner and/or thescanner head may be wireless, thereby simplifying handling and operationof the scanner and increasing accessibility under difficult scanningsituations, e.g. intra-oral or in the ear scanning. However, wirelessoperation may further increase the need for local data processing toavoid wireless transmission of raw 3D data.

The reference signal is provided by the pattern generating means and maybe periodic. The variation in the input signal may be periodic and itmay be confined to one or a few periods. The reference signal may bedetermined independently of the input signal. Specifically in the caseof a periodic variation, the phase between the oscillating input andreference signal may be known independently of the input signal. In thecase of a periodic variation the correlation is typically related to theamplitude of the variation. If the phase between the oscillating inputand reference signals is not known it is necessary to determine bothcosine and sinusoidal part of the input signal before the input signal'samplitude of variation can be determined. This is not necessary when thephase is known.

One way to define the correlation measure mathematically with a discreteset of measurements is as a dot product computed from a signal vector,I=(I₁, . . . , I_(n)), with n>1 elements representing sensor signals anda reference vector, f=(f₁, . . . , f_(n)), of same length as said signalvector of reference weights. The correlation measure A is then given by

$A = {{f \cdot I} = {\sum\limits_{i = 1}^{n}\;{f_{i}I_{i}}}}$

The indices on the elements in the signal vector represent sensorsignals that are recorded at different times and/or at differentsensors. In the case of a continuous measurement the above expression iseasily generalized to involve integration in place of the summation. Inthat case the integration parameter is time and/or one or more spatialcoordinates.

A preferred embodiment is to remove the DC part of the correlationsignal or correlation measure, i.e., when the reference vector elementssums to zero (Σ_(i=1) ^(n)f_(i)=0). The focus position can be found asan extremum of the correlation measure computed over all focus elementpositions. We note that in this case the correlation measure isproportional to the sample Pearson correlation coefficient between twovariables. If the DC part is not removed, there may exist a trend in DCsignal over all focus element positions, and this trend can bedominating numerically. In this situation, the focus position may stillbe found by analysis of the correlation measure and/or one or more ofits derivatives, preferably after trend removal.

Preferably, the global extremum should be found. However, artifacts suchas dirt on the optical system can result in false global maxima.Therefore, it can be advisable to look for local extrema in some cases.If the object being scanned is sufficiently translucent it may bepossible to identify interior surfaces or surface parts that areotherwise occluded. In such cases there may be several local extremathat corresponds to surfaces and it may be advantageous to processseveral or all extrema.

The correlation measure can typically be computed based on input signalsthat are available as digital images, i.e., images with a finite numberof discrete pixels. Therefore conveniently, the calculations forobtaining correlation measures can be performed for image pixels orgroups thereof. Correlation measures can then be visualized in aspseudo-images.

The correlation measure applied in this disclosure is inspired by theprinciple of a lock-in amplifier, in which the input signal ismultiplied by the reference signal and integrated over a specified time.In this disclosure, a reference signal is provided by the pattern.

Temporal Correlation

Temporal correlation involves a time-varying pattern. The light signalin the individual light sensing elements in the camera is recordedseveral times while the pattern configuration is varied. The correlationmeasure is thus at least computed with sensor signals recorded atdifferent times.

A principle to estimate light oscillation amplitude in a periodicallyvarying light signal is taught in WO 98/45745 where the amplitude iscalculated by first estimating a cosine and a sinusoidal part of thelight intensity oscillation. However, from a statistical point of viewthis is not optimal because two parameters are estimated to be able tocalculate the amplitude.

In this embodiment of the invention independent knowledge of the patternconfiguration at each light signal recording allows for calculating thecorrelation measure at each light sensing element.

In some embodiments of the invention the scanner comprises means forobtaining knowledge of the pattern configuration. To provide suchknowledge the scanner preferably further comprises means for registeringand/or monitoring the time varying pattern.

Each individual light sensing element, i.e. sensor element, in thecamera sees a variation in the light signal corresponding to thevariation of the light illuminating the object.

One embodiment of the invention obtains the time variation of thepattern by translating and/or rotating the pattern element. In this casethe pattern configuration may be obtained by means of a position encoderon the pattern element combined with prior knowledge of the patterngeometry that gives rise to a pattern variation across individualsensing elements. Knowledge of the pattern configuration thus arises asa combination of knowledge of the pattern geometry that results in avariation across different sensing elements and pattern registrationand/or monitoring during the 3D scan. In case of a rotating wheel as thepattern element the angular position of the wheel may then be obtainedby an encoder, e.g. mounted on the rim.

One embodiment of the invention involves a pattern that possessestranslational and/or rotational periodicity. In this embodiment there isa well-defined pattern oscillation period if the pattern issubstantially translated and/or rotated at a constant speed.

One embodiment of the invention comprises means for sampling each of aplurality of the sensor elements a plurality of times during one patternoscillation period, preferably sampled an integer number of times, suchas sampling 2, 3, 4, 5, 6, 7 or 8 times during each pattern oscillationperiod, thereby determining the light variation during a period.

The temporal correlation measure between the light variation and thepattern can be obtained by recording several images on the camera duringone oscillation period (or at least one oscillation period). The numberof images recorded during one oscillation period is denoted n. Theregistration of the pattern position for each individual image combinedwith the independently known pattern variation over all sensing element(i.e. obtaining knowledge of the pattern configuration) and the recordedimages allows for an efficient extraction of the correlation measure ineach individual sensing element in the camera. For a light sensingelement with label j, the n recorded light signals of that element aredenoted I_(1,j), . . . , I_(n,j). The correlation measure of thatelement, A_(j), may be expressed as

$A_{j} = {\sum\limits_{i = 1}^{n}\;{f_{i,j}I_{i,j}}}$

Here the reference signal or weight function f is obtained from theknowledge of the pattern configuration. f has two indices i,j. Thevariation of f with the first index is derived from the knowledge of thepattern position during each image recording. The variation of f withthe second index is derived from the knowledge of the pattern geometrywhich may be determined prior to the 3D scanning.

Preferably, but not necessarily, the reference signal f averages to zeroover time, i.e. for all j we have

${\sum\limits_{i = 1}^{n}\; f_{i,j}} = 0$to suppress the DC part of the light variation or correlation measure.The focus position corresponding to the pattern being in focus on theobject for a single sensor element in the camera will be given by anextremum value of the correlation measure of that sensor element whenthe focus position is varied over a range of values. The focus positionmay be varied in equal steps from one end of the scanning region to theother.

To obtain a sharp image of an object by means of a camera the objectmust be in focus and the optics of the camera and the object must be ina fixed spatial relationship during the exposure time of the imagesensor of the camera. Applied to the present disclosure this shouldimply that the pattern and the focus should be varied in discrete stepsto be able to fix the pattern and the focus for each image sampled inthe camera, i.e. fixed during the exposure time of the sensor array.However, to increase the sensitivity of the image data the exposure timeof the sensor array should be as high as the sensor frame rate permits.Thus, in the preferred embodiment of the invention images are recorded(sampled) in the camera while the pattern is continuously varying (e.g.by continuously rotating a pattern wheel) and the focus plane iscontinuously moved. This implies that the individual images will beslightly blurred since they are the result of a time-integration of theimage while the pattern is varying and the focus plane is moved. This issomething that one could expect to lead to deterioration of the dataquality, but in practice the advantage of concurrent variation of thepattern and the focus plane is bigger than the drawback.

In another embodiment of the invention images are recorded (sampled) inthe camera while the pattern is fixed and the focus plane iscontinuously moved, i.e. no movement of the pattern. This could be thecase when the light source is a segmented light source, such as asegment LED that flashes in an appropriate fashion. In this embodimentthe knowledge of the pattern is obtained by a combination of priorknowledge of the geometry of the individual segments on the segmentedLED give rise to a variation across light sensing elements and theapplied current to different segments of the LED at each recording.

In yet another embodiment of the invention images are recorded (sampled)in the camera while the pattern is continuously varying and the focusplane is fixed.

In yet another embodiment of the invention images are recorded (sampled)in the camera while the pattern and the focus plane are fixed.

The temporal correlation principle may be applied in general withinimage analysis. Thus, a further embodiment of the invention relates to amethod for calculating the amplitude of a light intensity oscillation inat least one (photoelectric) light sensitive element, said lightintensity oscillation generated by a periodically varying illuminationpattern and said amplitude calculated in at least one patternoscillation period, said method comprising the steps of:

-   -   providing the following a predetermined number of sampling times        during a pattern oscillation period:        -   sampling the light sensitive element thereby providing the            signal of said light sensitive element, and        -   providing an angular position and/or a phase of the            periodically varying illumination pattern for said sampling,            and    -   calculating said amplitude(s) by integrating the products of a        predetermined periodic function and the signal of the        corresponding light sensitive element over said predetermined        number of sampling times, wherein said periodic function is a        function of the angular position and/or the phase of the        periodically varying illumination pattern.

This may also be expressed as

$A = {\sum\limits_{i}{{f\left( p_{i} \right)}I_{i}}}$where A is the calculated amplitude or correlation measure, i is theindex for each sampling, f is the periodic function, p_(i) is theangular position/phase of the illumination pattern for sampling i andI_(i) is the signal of the light sensitive element for sampling i.Preferably the periodic function averages to zero over a patternoscillation period, i.e. Σ_(i)f(p_(i))=0.

To generalize the principle to a plurality of light sensitive elements,for example in a sensor array, the angular position/phase of theillumination pattern for a specific light sensitive element may consistof an angular position/phase associated with the illumination patternplus a constant offset associated with the specific light sensitiveelement. Thereby the correlation measure or amplitude of the lightoscillation in light sensitive element j may be expressed as

${A_{j} = {\sum\limits_{i}{{f\left( {\theta_{j} + p_{i}} \right)}I_{i,j}}}},$

where θ_(j) is the constant offset for light sensitive element j.

A periodically varying illumination pattern may be generated by arotating wheel with an opaque mask comprising a plurality of radialspokes arranged in a symmetrical order. The angular position of thewheel will thereby correspond to the angular position of the pattern andthis angular position may obtained by an encoder mounted on the rim ofthe wheel. The pattern variation across different sensor elements fordifferent position of the pattern may be determined prior to the 3Dscanning in a calibration routine. A combination of knowledge of thispattern variation and the pattern position constitutes knowledge of thepattern configuration. A period of this pattern may for example be thetime between two spokes and the amplitude of a single or a plurality oflight sensitive elements of this period may be calculated by samplinge.g. four times in this period.

A periodically varying illumination pattern may generated by a Ronchiruling moving orthogonal to the lines and the position is measured by anencoder. This position corresponds to the angular position of thegenerated pattern. Alternatively, a checkerboard pattern could be used.

A periodically varying illumination pattern may generated by aone-dimensional array of LEDs that can be controlled line wise.

A varying illumination pattern may generated by a LCD or DLP basedprojector.

Optical Correlation

The abovementioned correlation principle (temporal correlation) requiressome sort of registering of the time varying pattern, e.g. knowledge ofthe pattern configuration at each light level recording in the camera.However, a correlation principle without this registering may beprovided in another embodiment of the invention. This principle istermed “optical correlation”.

In this embodiment of the invention an image of the pattern itself andan image of at least a part of the object being scanned with the patternprojected onto it is combined on the camera. I.e. the image on thecamera is a superposition of the pattern itself and the object beingprobed with the pattern projected onto it. A different way of expressingthis is that the image on the camera substantially is a multiplicationof an image of the pattern projected onto the object with the patternitself.

This may be provided in the following way. In a further embodiment ofthe invention the pattern generation means comprises a transparentpattern element with an opaque mask. The probe light is transmittedthrough the pattern element, preferably transmitted transversely throughthe pattern element. The light returned from the object being scanned isretransmitted the opposite way through said pattern element and imagedonto the camera. This is preferably done in a way where the image of thepattern illuminating the object and the image of the pattern itself arecoinciding when both are imaged onto the camera. One particular exampleof a pattern is a rotating wheel with an opaque mask comprising aplurality of radial spokes arranged in a symmetrical order such that thepattern possesses rotational periodicity. In this embodiment there is awell-defined pattern oscillation period if the pattern is substantiallyrotated at a constant speed. We define the oscillation period as 2□/□□

We note that in the described embodiment of the invention theillumination pattern is a pattern of light and darkness. A light sensingelement in the camera with a signal proportional to the integrated lightintensity during the camera integration time □t with label j, I_(j) isgiven by

I_(j) = K∫_(t)^(t + δ t)T_(j)(t^(′))S_(j)(t^(′))dt^(′)

Here K is the proportionality constant of the sensor signal, t is thestart of the camera integration time, T_(j) is the time-varyingtransmission of the part of the rotating pattern element imaged onto thejth light sensing element, and S_(j) is the time-varying light intensityof light returned from the scanned object and imaged onto the jth lightsensing element. In the described embodiment T_(j) is the step functionsubstantially defined by T_(j)(t)=0 for sin(□t+□_(j))>0 and T_(j)(t)=1elsewhere. □_(j) is a phase dependent on the position of the jth imagingsensor.

The signal on the light sensing element is a correlation measure of thepattern and the light returned from the object being scanned. Thetime-varying transmission takes the role of the reference signal and thetime-varying light intensity of light returned from the scanned objecttakes the role of the input signal. The advantage of this embodiment ofthe invention is that a normal CCD or CMOS camera with intensity sensingelements may be used to record the correlation measure directly sincethis appears as an intensity on the sensing elements. Another way ofexpressing this is that the computation of the correlation measure takesplace in the analog, optical domain instead of in an electronic domainsuch as an FPGA or a PC.

The focus position corresponding to the pattern being in focus on theobject being scanned for a single sensor element in the camera will thenbe given by the maximum value of the correlation measure recorded withthat sensor element when the focus position is varied over a range ofvalues. The focus position may be varied in equal steps from one end ofthe scanning region to the other. One embodiment of the inventioncomprises means for recording and/or integrating and/or monitoringand/or storing each of a plurality of the sensor elements over a rangeof focus plane positions.

Preferably, the global maximum should be found. However, artifacts suchas dirt on the optical system can result in false global maxima.Therefore, it can be advisable to look for local maxima in some cases.

Since the reference signal does not average to zero the correlationmeasure has a DC component. Since the DC part is not removed, there mayexist a trend in DC signal over all focus element positions, and thistrend can be dominating numerically. In this situation, the focusposition may still be found by analysis of the correlation measureand/or one or more of its derivatives.

In a further embodiment of the invention the camera integration time isan integer number M of the pattern oscillation period, i.e. □t=2□M/□.One advantage of this embodiment is that the magnitude of thecorrelation measure can be measured with a better signal-to-noise ratioin the presence of noise than if the camera integration time is not aninteger number of the pattern oscillation period.

In another further embodiment of the invention the camera integrationtime is much longer than pattern oscillation period, i.e. □t>>2□M/□.Many times the pattern oscillation time would here mean e.g. cameraintegration time at least 10 times the oscillation time or morepreferably such as at least 100 or 1000 times the oscillation time. Oneadvantage of this embodiment is that there is no need forsynchronization of camera integration time and pattern oscillation timesince for very long camera integration times compared to the patternoscillation time the recorded correlation measure is substantiallyindependent of accurate synchronization.

Equivalent to the temporal correlation principle the optical correlationprinciple may be applied in general within image analysis. Thus, afurther embodiment of the invention relates to a method for calculatingthe amplitude of a light intensity oscillation in at least one(photoelectric) light sensitive element, said light intensityoscillation generated by a superposition of a varying illuminationpattern with itself, and said amplitude calculated by time integratingthe signal from said at least one light sensitive element over aplurality of pattern oscillation periods.

Spatial Correlation

The above mentioned correlation principles (temporal correlation andoptical correlation) require the pattern to be varying in time. If theoptical system and camera provides a lateral resolution which is atleast two times what is needed for the scan of the object then it ispossible to scan with a static pattern, i.e. a pattern which is notchanging in time. This principle is termed “spatial correlation”. Thecorrelation measure is thus at least computed with sensor signalsrecorded at different sensor sites.

The lateral resolution of an optical system is to be understood as theability of optical elements in the optical system, e.g. a lens system,to image spatial frequencies on the object being scanned up to a certainpoint. Modulation transfer curves of the optical system are typicallyused to describe imaging of spatial frequencies in an optical system.One could e.g. define the resolution of the optical system as thespatial frequency on the object being scanned where the modulationtransfer curve has decreased to e.g. 50%. The resolution of the camerais a combined effect of the spacing of the individual camera sensorelements and the resolution of the optical system.

In the spatial correlation the correlation measure refers to acorrelation between input signal and reference signal occurring in spacerather than in time. Thus, in one embodiment of the invention theresolution of the measured 3D geometry is equal to the resolution of thecamera. However, for the spatial correlation the resolution of themeasured 3D geometry is lower than the resolution of the camera, such asat least 2 times lower, such as at least 3 times lower, such as at least4 times lower, such as least 5 times lower, such as at least 10 timeslower. The sensor element array is preferably divided into groups ofsensor elements, preferably rectangular groups, such as square groups ofsensor elements, preferably adjacent sensor elements. The resolution ofthe scan, i.e. the measured 3D geometry, will then be determined by thesize of these groups of sensor elements. The oscillation in the lightsignal is provided within these groups of sensor elements, and theamplitude of the light oscillation may then be obtained by analyzing thegroups of sensor elements. The division of the sensor element array intogroups is preferably provided in the data processing stage, i.e. thedivision is not a physical division thereby possibly requiring aspecially adapted sensor array. Thus, the division into groups is“virtual” even though the single pixel in a group is an actual physicalpixel.

In one embodiment of the invention the pattern possesses translationalperiodicity along at least one spatial coordinate. In a furtherembodiment of the invention the spatially periodic pattern is alignedwith the rows and/or the columns of the array of sensor elements. Forexample in the case of a static line pattern the rows or columns of thepixels in the camera may be parallel with the lines of the pattern. Orin the case of a static checkerboard pattern the row and columns of thecheckerboard may be aligned with the rows and columns, respectively, ofthe pixels in the camera. By aligning is meant that the image of thepattern onto the camera is aligned with the “pattern” of the sensorelement in the sensor array of the camera. Thus, a certain physicallocation and orientation of the pattern generation means and the camerarequires a certain configuration of the optical components of thescanner for the pattern to be aligned with sensor array of the camera.

In a further embodiment of the invention at least one spatial period ofthe pattern corresponds to a group of sensor elements. In a furtherembodiment of the invention all groups of sensor elements contain thesame number of elements and have the same shape. E.g. when the period ofa checkerboard pattern corresponds to a square group of e.g. 2×2, 3×3,4×4, 5×5, 6×6, 7×7, 8×8, 9×9, 10×10 or more pixels on the camera.

In yet another embodiment one or more edges of the pattern is alignedwith and/or coincide with one or more edges of the array of sensorelements. For example a checkerboard pattern may be aligned with thecamera pixels in such a way that the edges of the image of thecheckerboard pattern onto the camera coincide with the edges of thepixels.

In spatial correlation independent knowledge of the patternconfiguration allows for calculating the correlation measure at eachgroup of light sensing. For a spatially periodic illumination thiscorrelation measure can be computed without having to estimate thecosine and sinusoidal part of the light intensity oscillation. Theknowledge of the pattern configuration may be obtained prior to the 3Dscanning.

In a further embodiment of the invention the correlation measure, A_(j),within a group of sensor elements with label j is determined by means ofthe following formula:

$A_{j} = {\sum\limits_{i = 1}^{n}\;{f_{i,j}I_{i,j}}}$

Where n is the number of sensor elements in a group of sensors,f_(j)=(f_(1,j), . . . f_(n,j)) is the reference signal vector obtainedfrom knowledge of the pattern configuration, and I_(j)=(I_(1,j), . . .I_(n,j)) is input signal vector. For the case of sensors grouped insquare regions with N sensors as square length then n=N².

Preferably, but not necessarily, the elements of the reference signalvector averages to zero over space, i.e. for all j we have

${\sum\limits_{i = 1}^{n}\; f_{i,j}} = 0$to suppress the DC part of the correlation measure. The focus positioncorresponding to the pattern being in focus on the object for a singlegroup of sensor elements in the camera will be given by an extremumvalue of the correlation measure of that sensor element group when thefocus position is varied over a range of values. The focus position maybe varied in equal steps from one end of the scanning region to theother.

In the case of a static checkerboard pattern with edges aligned with thecamera pixels and with the pixel groups having an even number of pixelssuch as 2×2, 4×4, 6×6, 8×8, 10×10, a natural choice of the referencevector f would be for its elements to assume the value 1 for the pixelsthat image a bright square of the checkerboard and −1 for the pixelsthat image a dark square of the checkerboard.

Equivalent to the other correlation principles the spatial correlationprinciple may be applied in general within image analysis. In particularin a situation where the resolution of the camera is higher than what isnecessary in the final image. Thus, a further embodiment of theinvention relates to a method for calculating the amplitude(s) of alight intensity oscillation in at least one group of light sensitiveelements, said light intensity oscillation generated by a spatiallyvarying static illumination pattern, said method comprising the stepsof:

-   -   providing the signal from each light sensitive element in said        group of light sensitive elements, and    -   calculating said amplitude(s) by integrating the products of a        predetermined function and the signal from the corresponding        light sensitive element over said group of light sensitive        elements, wherein said predetermined function is a function        reflecting the illumination pattern.

To generalize the principle to a plurality of light sensitive elements,for example in a sensor array, the correlation measure or amplitude ofthe light oscillation in group j may be expressed as

${A_{j} = {\sum\limits_{i = 1}^{n}\;{{f\left( {i,j} \right)}I_{i,j}}}},$where n is the number of sensor elements in group j, I_(i,j) is thesignal from the ith sensor element in group j and f(i,j) is apredetermined function reflecting the pattern.

Compared to temporal correlation, spatial correlation has the advantagethat no moving pattern is required. This implies that knowledge of thepattern configuration may be obtained prior to the 3D scanning.Conversely, the advantage of temporal correlation is its higherresolution, as no pixel grouping is required.

All correlation principles, when embodied with an image sensor thatallows very high frame rates, enable 3D scanning of objects in motionwith little motion blur. It also becomes possible to trace movingobjects over time (“4D scanning”), with useful applications for examplein machine vision and dynamic deformation measurement. Very high framerates in this context are at least 500, but preferably at least 2000frames per second.

Transforming Correlation Measure Extrema to 3D World Coordinates

Relating identified focus position(s) for camera sensor or camera sensorgroups to 3D world coordinates may be done by ray tracing through theoptical system. Before such ray tracing can be performed the parametersof the optical system need to be known. One embodiment of the inventioncomprises a calibration step to obtain such knowledge. A furtherembodiment of the invention comprises a calibration step in which imagesof an object of known geometry are recorded for a plurality of focuspositions. Such an object may be a planar checkerboard pattern. Then,the scanner can be calibrated by generating simulated ray traced imagesof the calibration object and then adjusting optical system parametersas to minimize the difference between the simulated and recorded images.

In a further embodiment of the invention the calibration step requiresrecording of images for a plurality of focus positions for severaldifferent calibration objects and/or several different orientationsand/or positions of one calibration object.

With knowledge of the parameters of the optical system, one can employbackward ray tracing technique to estimate the 2D−>3D mapping. Thisrequires that the scanner's optical system be known, preferably throughcalibration. The following steps can be performed:

1. From each pixel of the image (at the image sensor), trace a certainnumber of rays, starting from the image sensor and through the opticalsystem (backward ray tracing).

2. From the rays that emit, calculate the focus point, the point whereall these rays substantially intersect. This point represents the 3Dcoordinate of where a 2D pixel will be in focus, i.e., in yield theglobal maximum of light oscillation amplitude.

3. Generate a look up table for all the pixels with their corresponding3D coordinates. The above steps are repeated for a number of differentfocus lens positions covering the scanner's operation range.

Specular Reflections

High spatial contrast of the in-focus pattern image on the object isoften necessary to obtain a good signal to noise ratio of thecorrelation measure on the camera. This in turn may be necessary toobtain a good estimation of the focus position corresponding to anextremum in the correlation measure. This sufficient signal to noiseratio for successful scanning is often easily achieved in objects with adiffuse surface and negligible light penetration. For some objects,however, it is difficult to achieve high spatial contrast.

A difficult kind of object, for instance, is an object displayingmultiple scattering of the incident light with a light diffusion lengthlarge compared to the smallest feature size of the spatial patternimaged onto the object. A human tooth is an example of such an object.The human ear and ear canal are other examples. In case of intra oralscanning, the scanning should preferably be provided without sprayingand/or drying the teeth to reduce the specular reflections and lightpenetration. Improved spatial contrast can be achieved by preferentialimaging of the specular surface reflection from the object on thecamera. Thus, one embodiment of the invention comprises means forpreferential/selectively imaging of specular reflected light and/ordiffusively reflected light. This may be provided if the scanner furthercomprises means for polarizing the probe light, for example by means ofat least one polarizing beam splitter. A polarizing beam splitter mayfor instance be provided for forming an image of the object in thecamera. This may be utilized to extinguish specular reflections, becauseif the incident light is linearly polarized a specular reflection fromthe object has the property that it preserves its polarization state

The scanner according to the invention may further comprise means forchanging the polarization state of the probe light and/or the lightreflected from the object. This can be provided by means of aretardation plate, preferably located in the optical path. In oneembodiment of the invention the retardation plate is a quarter waveretardation plate. A linearly polarized light wave is transformed into acircularly polarized light wave upon passage of a quarter wave platewith an orientation of 45 degrees of its fast axis to the linearpolarization direction. This may be utilized to enhance specularreflections because a specular reflection from the object has theproperty that it flips the helicity of a circularly polarized lightwave, whereas light that is reflected by one or more scattering eventsbecomes depolarized.

The Field of View (Scanning Length)

In one embodiment of the invention the probe light is transmittedtowards the object in a direction substantially parallel with theoptical axis. However, for the scan head to be entered into a smallspace such as the oral cavity of a patient it is necessary that the tipof the scan head is sufficiently small. At the same time the light outof the scan head need to leave the scan head in a direction differentfrom the optical axis. Thus, a further embodiment of the inventioncomprises means for directing the probe light and/or imaging an objectin a direction different from the optical axis. This may be provided bymeans of at least one folding element, preferably located along theoptical axis, for directing the probe light and/or imaging an object ina direction different from the optical axis. The folding element couldbe a light reflecting element such as a mirror or a prism. In oneembodiment of the invention a 45 degree mirror is used as folding opticsto direct the light path onto the object. Thereby the probe light isguided in a direction perpendicular to the optical axis. In thisembodiment the height of the scan tip is at least as large as the scanlength and preferably of approximately equal size.

One embodiment of the invention comprises at least two light sources,such as light sources with different wavelengths and/or differentpolarization. Preferably also control means for controlling said atleast two light sources. Preferably this embodiment comprises means forcombining and/or merging light from said at least two light sources.Preferably also means for separating light from said at least two lightsources. If waveguide light sources are used they may be merged bywaveguides. However, one or more diffusers may also be provided to mergelight sources.

Separation and/or merging may be provided by at least one optical devicewhich is partially light transmitting and partially light reflecting,said optical device preferably located along the optical axis, anoptical device such as a coated mirror or coated plate. One embodimentcomprises at least two of said optical devices, said optical devicespreferably displaced along the optical axis. Preferably at least one ofsaid optical devices transmits light at certain wavelengths and/orpolarizations and reflects light at other wavelengths and/orpolarizations.

One exemplary embodiment of the invention comprises at least a first anda second light source, said light sources having different wavelengthand/or polarization, and wherein

-   -   a first optical device reflects light from said first light        source in a direction different from the optical axis and        transmits light from said second light source, and    -   a second optical device reflects light from said second light        source in a direction different from the optical axis.        Preferably said first and second optical devices reflect the        probe light in parallel directions, preferably in a direction        perpendicular to the optical axis, thereby imaging different        parts of the object surface. Said different parts of the object        surface may be at least partially overlapping.

Thus, for example light from a first and a second light source emittinglight of different wavelengths (and/or polarizations) is merged togetherusing a suitably coated plate that transmits the light from the firstlight source and reflects the light from the second light source. At thescan tip along the optical axis a first optical device (e.g. a suitablycoated plate, dichroic filter) reflects the light from the first lightsource onto the object and transmits the light from the second lightsource to a second optical device (e.g. a mirror) at the end of the scantip, i.e. further down the optical axis. During scanning the focusposition is moved such that the light from the first light source isused to project an image of the pattern to a position below the firstoptical device while second light source is switched off. The 3D surfaceof the object in the region below the first optical device is recorded.Then the first light source is switched off and the second light sourceis switched on and the focus position is moved such that the light fromthe second light source is used to project an image of the pattern to aposition below the second optical device. The 3D surface of the objectin the region below the second optical device is recorded. The regioncovered with the light from the two light sources respectively maypartially overlap.

In another embodiment of the invention the probe light is directed in adirection different from the optical axis by means of a curved foldmirror. This embodiment may comprise one or more optical elements, suchas lenses, with surfaces that may be aspherical to provide correctedoptical imaging.

A further embodiment of the invention comprises of at least onetranslation stage for translating mirror(s) along the optical axis. Thisallows for a scan tip with a smaller height than the scan length. Alarge scan length can be achieved by combining several scans with themirror(s) in different positions along the optical axis.

In another embodiment of the invention the probe light is directed in adirection different from the optical axis by means of at least onegrating that provides anamorphic magnification so that the image of thepattern on the object being scanned is stretched. The grating may beblazed. In this embodiment the light source needs to be monochromatic orsemi-monochromatic.

The abovementioned embodiments suitable for increasing the scan lengthmay comprise control means for providing a coordination of the lightsources and the focus element.

Color Scanning

One embodiment of the invention is only registering the surface topology(geometry) of the object being scanned. However, another embodiment ofthe invention is being adapted to obtain the color of the surface beingscanned, i.e. capable of registering the color of the individual surfaceelements of the object being scanned together with the surface topologyof the object being scanned. To obtain color information the lightsource needs to be white or to comprise at least three monochromaticlight sources with colors distributed across the visible part of theelectromagnetic spectrum.

To provide color information the array of sensor elements may be a colorimage sensor. The image sensor may accommodate a Bayer color filterscheme. However, other color image sensor types may be provided, such asa Foveon type color image sensor, wherein the image sensor providescolor registration in each sensor element.

One embodiment of the invention comprises means selecting one color ofthe probe light at a time, i.e. selectively switching between differentcolors of the probe light, thereby illuminating the object withdifferent colors. If a white light source is used then some kind ofcolor filtering must be provided. Preferably comprising a plurality ofcolor filters, such as red, green and blue color filters, and means forinserting said color filters singly in front of the white light source,thereby selecting a color of the probe light.

In one embodiment of the invention color filters are integrated in thepattern generation means, i.e. the pattern generation means comprisescolor filters, such as translucent and/or transparent parts that aresubstantially monochromatically colored. For example a pattern elementsuch as a rotating wheel with an opaque mask and where thetranslucent/transparent parts are color filters. For example one thirdof the wheel is red, one third is green and one third is blue.

Probe light of different colors may also be provided by at least threemonochromatic light sources, such as lasers or LED's, said light sourceshaving wavelengths distributed across the visible part of the wavelengthspectrum. This will in general also require means for merging said lightsources. For example suitable coated plates. In the case of waveguidelight sources, the merging may be provided by a waveguide element.

To handle the different colors of the probe light the optical system ispreferably substantially achromatic. One embodiment of the inventioncomprises means for switching between at least two colors, preferablythree colors, such as red, green and blue, of the probe light for afocal plane position. I.e. for a single focal plane position it ispossible to switch between different colors of the probe light. Forexample by switching on and off different monochromatic light sources(having one only light source turned on at a time) or by applyingdifferent color filters. Furthermore, the amplitude of the light signalof each of a plurality of the sensor elements may be determined for eachcolor for each focal plane positions. I.e. for each focus position thecolor of the probe light may be switched. The embedded time varyingpattern provides a single color oscillating light signal and theamplitude of the signal in each sensor element may be determined forthat color.

Switching to the next color the amplitude may be determined again. Whenthe amplitude has been determined for all colors the focus position ischanged and the process is repeated. The color of the surface beingscanned may then be obtained by combining and/or weighing the colorinformation from a plurality of the sensor elements. E.g. the colorexpressed as e.g. an RGB color coordinate of each surface element can bereconstructed by appropriate weighting of the amplitude signal for eachcolor corresponding to the maximum amplitude. This technique may also beapplied when a static pattern is provided where the color of at least apart of the pattern is varying in time.

To decrease the amount of data to be processed the color resolution ofthe imaging may be chosen to be less than the spatial resolution. Thecolor information is then provided by data interpolation. Thus, in oneembodiment of the invention the amplitude of the light signal of each ofa plurality of the sensor elements is determined for each color forselected full color focal plane positions, and the amplitude of thelight signal of each of a plurality of the sensor elements is determinedfor one color for each focal plane position. Then the color of thesurface being scanned may be obtained by interpolating the colorinformation from full color focal plane positions. Thus, for example theamplitude is registered for all colors at an interval of N focuspositions; while one color is selected for determination of theamplitude at all focus positions. N is a number which could be e.g. 3,5, or 10. This results in a color resolution which is less than theresolution of the surface topology. This technique may also be appliedwhen a static pattern is provided where the color of at least a part ofthe pattern is varying in time.

Another embodiment of the invention does not register full colorinformation and employs only two light sources with different colors. Anexample of this is a dental scanner that uses red and blue light todistinguish hard (tooth) tissue from soft (gum) tissue.

Impression Scanning

One embodiment of the invention is adapted to impression scanning, suchas scanning of dental impressions and/or ear canal impressions.

Small Cavity Scanner

Specific applications of the scanner according to the invention relatesto scanning of cavities, in particular body cavities. Scanning incavities may relate to scanning of objects in the cavity, such asscanning of teeth in a mouth. However, scanning of e.g. the ear relateto scanning of the inner surface of the cavity itself. In generalscanning of a cavity, especially a small cavity, requires some kind ofprobe for the scanner. Thus, in one embodiment of the invention thepoint of emission of probe light and the point of accumulation ofreflected light is located on a probe, said probe being adapted to beentered into a cavity, such as a body cavity.

In another embodiment of the invention the probe is adapted to scan atleast a part of the surface of a cavity, such as an ear canal. Theability to scan at least a part of the external part of the ear and/orthe ear canal and make a virtual or real model of the ear is essentialin the design of modern custom-fitted hearing aid (e.g. ear shell ormold). Today, scanning of ears is performed in a two-step process wherea silicone impression of the ear is taken first and the impression issubsequently scanned using an external scanner in a second step.

Thus, one embodiment of the invention comprises

-   -   a housing accommodating the camera, pattern generation means,        focus varying means and data processing means, and    -   at least one probe accommodating a first optical system,        preferably a substantially elongated probe.

Preferably, the point of emission of probe light and the point ofaccumulation of light returned from the scanned object is located onsaid probe. The optical system in the probe is for transmitting theprobe light from the housing toward the object and also for transmittingand/or imaging light returned from the object back towards the housingwhere the camera is located. Thus, the optical system in the probe maycomprise a system of lenses. In one embodiment of the invention probemay comprise at least one optical fibre and/or a fibre bundle fortransmitting/transporting/guiding the probe light and/or the returnedlight from the object surface. In this case the optical fibre(s) may actas an optical relay system that merely transports light (i.e. probelight and returned light) inside the probe. In one embodiment of theinvention the probe is endoscopic. The probe may be rigid or flexible.Use of optical fibre(s) in the probe may e.g. provide a flexible probewith a small diameter.

In one embodiment of the invention the light is transmitted to theobject and imaged by means of only the optical system in the probe, thefirst optical system. However, in a further embodiment of the inventionthe housing may further comprise a second optical system.

In a further embodiment of the invention the probe is detachable fromthe housing. Then preferably a first point of emission of probe lightand a first point of accumulation of returned light is located on theprobe, and a second point of emission of probe light and a second pointof accumulation of returned light is located on the housing. This mayrequire optical systems in both the housing and the probe. Thus, a scanmay be obtained with the probe attached to the housing. However, a scanmay also be obtained with the probe detached from the housing, i.e. thehousing may be a standalone scanner in itself. For example the probe maybe adapted to be inserted into and scanning the inside of a cavity,whereas the housing may be adapted to scanning of exterior surfaces. Theattachment of the probe may include mechanical and/or electricaltransfer between the housing and the probe. For instance attaching theprobe may provide an electrical signal to the control electronics in thehousing that signals the current configuration of the device.

In one embodiment of the invention the probe light is directed towardthe object in a direction substantially parallel with the optical axisand/or the longitudinal axis of the probe. In a further embodiment theprobe comprises a posterior reflective element, such as a mirror, fordirecting the probe light in a direction different from the opticalaxis, preferably in a direction perpendicular to the optical axis.Applying to the abovementioned example with a stand-alone scannerhousing with the probe detached, the probe light may exit the housing ina direction parallel with the optical axis of the optical system in thehousing (i.e. the second optical system), whereas with the probeattached the probe light may be directed in a direction different thanthe optical axis of the optical system of the probe (i.e. the firstoptical system). Thereby the probe is better adapted to scanning acavity.

In some embodiments of this invention, waste heat generated in thescanner is used to warm the probe such that no or less condensationoccurs on the probe when the probe is inside the body cavity, e.g. themouth. Waste heat can, e.g., be generated by the processing electronics,the light source, and/or the mechanism that moves the focus element.

In some embodiments of this invention, the scanner provides feedback tothe user when the registration of subsequent scans to a larger model ofthe 3D surface fails. For example, the scanner could flash the lightsource.

Further, the probe may comprise means for rotating/spinning thereflective element, preferably around an axis substantially parallelwith the optical axis and/or the longitudinal axis of the probe. Therebythe probe may be adapted to provide a scan 360° around the optical axisand/or the longitudinal axis of the probe, preferably without rotationof probe and/or scanner.

In a further embodiment of the invention a plurality of different probesmatches the housing. Thereby different probes adapted to differentenvironments, surfaces, cavities, etc. may be attached to the housing toaccount for different scanning situations. A specific example of this iswhen the scanner comprises a first probe being adapted to scan theinterior part of a human ear and a second probe being adapted to scanthe exterior part of said human ear. Instead of a second probe it may bethe housing itself, i.e. with the probe detached, that is adapted toscan the exterior part of said human ear. I.e. the housing may beadapted to perform a 3D surface scan. In other words: the housing withthe probe attached may be adapted to scan the interior part of a humanear and the housing with the probe detached may be adapted to scan theexterior part of said human ear. Preferably, means for merging and/orcombining 3D data for the interior and exterior part of the earprovided, thereby providing a full 3D model of a human ear.

For handheld embodiments of this invention, a pistol-like design isergonomic because the device rests comfortably inside the hand of theoperator, with most of the mass resting on top of the hand and/or wrist.In such a design, it is advantageous to be able to orient theabove-mentioned posterior reflective in multiple positions. For example,it could be possible to rotate a probe with the posterior reflectiveelement, with or without the step of detaching it from the main body ofthe scanning device. Detachable probes may also be autoclavable, whichis a definitely advantage for scanners applied in humans, e.g., asmedical devices. For embodiments of this invention that realize aphysically moving focus element by means of a motor, it is advantageousto place this motor inside a grip of the pistol-like shape.

Use of Motion, Gravity, and Magnetic Sensors

Handheld embodiments of the invention preferably include motion sensorssuch as accelerometers and/or gyros. Preferably, these motion sensorsare small like microelectromechanical systems (MEMS) motion sensors. Themotion sensors should preferably measure all motion in 3D, i.e., bothtranslations and rotations for the three principal coordinate axes. Thebenefits are:

-   -   A) Motion sensors can detect vibrations and/or shaking. Scans        such affected can be either discarded or corrected by use of        image stabilization techniques.    -   B) Motion sensors can help with stitching and/or registering        partial scans to each other. This advantage is relevant when the        field of view of the scanner is smaller than the object to be        scanned. In this situation, the scanner is applied for small        regions of the object (one at a time) that then are combined to        obtain the full scan. In the ideal case, motion sensors can        provide the required relative rigid-motion transformation        between partial scans' local coordinates, because they measure        the relative position of the scanning device in each partial        scan. Motion sensors with limited accuracy can still provide a        first guess for a software-based stitching/registration of        partial scans based on, e.g., the Iterative Closest Point class        of algorithms, resulting in reduced computation time.    -   C) Motion sensors can be used (also) as a remote control for the        software that accompanies the invention. Such software, for        example, can be used to visualize the acquired scan. With the        scanner device now acting as a remote control, the user can, for        example, rotate and/or pan the view (by moving the remote        control in the same way as the object on the computer screen        should “move”). Especially in clinical application, such dual        use of the handheld scanner is preferable out of hygienic        considerations, because the operator avoids contamination from        alternative, hand-operated input devices (touch screen, mouse,        keyboard, etc).

Even if it is too inaccurate to sense translational motion, a 3-axisaccelerometer can provide the direction of gravity relative to thescanning device. Also a magnetometer can provide directional informationrelative to the scanning device, in this case from the earth's magneticfield. Therefore, such devices can help with stitching/registration andact as a remote control element.

The present invention relates to different aspects including the scannerdevice described above and in the following, and corresponding methods,devices, uses and/or product means, each yielding one or more of thebenefits and advantages described in connection with the first mentionedaspect, and each having one or more embodiments corresponding to theembodiments described in connection with the first mentioned aspectand/or disclosed in the appended claims.

In particular, disclosed herein is a method for obtaining and/ormeasuring the 3D geometry of at least a part of the surface of anobject, said method comprising the steps of:

-   -   generating a probe light incorporating a spatial pattern,    -   transmitting the probe light towards the object along the        optical axis of an optical system, thereby illuminating at least        a part of the object with said pattern,    -   transmitting at least a part of the light returned from the        object to the camera,    -   varying the position of the focus plane of the pattern on the        object while maintaining a fixed spatial relation of the scanner        and the object,    -   obtaining at least one image from said array of sensor elements,    -   evaluating a correlation measure at each focus plane position        between at least one image pixel and a weight function, where        the weight function is determined based on information of the        configuration of the spatial pattern;    -   determining by analysis of the correlation measure the in-focus        position(s) of:        -   each of a plurality of image pixels in the camera for said            range of focus plane positions, or        -   each of a plurality of groups of image pixels in the camera            for said range of focus planes, and    -   transforming in-focus data into 3D real world coordinates.

Disclosed is also a computer program product comprising program codemeans for causing a data processing system to perform the method, whensaid program code means are executed on the data processing system.

Disclosed is also a computer program product, comprising acomputer-readable medium having stored there on the program code means.

Another aspect of the invention relates to a scanner for obtainingand/or measuring the 3D geometry of at least a part of the surface of anobject, said scanner comprising:

-   -   at least one camera accommodating an array of sensor elements,    -   means for generating a probe light,    -   means for transmitting the probe light towards the object        thereby illuminating at least a part of the object,    -   means for transmitting light returned from the object to the        camera,    -   means for varying the position of the focus plane on the object,    -   means for obtaining at least one image from said array of sensor        elements,    -   means for:        -   a) determining the in-focus position(s) of:            -   each of a plurality of the sensor elements for a range                of focus plane positions, or            -   each of a plurality of groups of the sensor elements for                a range of focus plane positions, and        -   b) transforming in-focus data into 3D real world            coordinates;    -   wherein the scanner further comprises counter-weight means for        counter-balancing the means for varying the position of the        focus plane.

Disclosed is also a method for obtaining and/or measuring the 3Dgeometry of at least a part of the surface of an object, said methodcomprising the steps of:

-   -   accommodating an array of sensor elements,    -   generating a probe light,    -   transmitting the probe light towards the object thereby        illuminating at least a part of the object,    -   transmitting light returned from the object to the camera,    -   varying the position of the focus plane on the object,    -   obtaining at least one image from said array of sensor elements,    -   determining the in-focus position(s) of:        -   each of a plurality of the sensor elements for a range of            focus plane positions, or        -   each of a plurality of groups of the sensor elements for a            range of focus plane positions, and    -   transforming in-focus data into 3D real world coordinates;    -   wherein the method further comprises counter-balancing the means        for varying the position of the focus plane.

Another aspect of the invention relates to a handheld 3D scanner with agrip at an angle of more than 30 degrees from the scanner's main opticalaxis, for use in intraoral or in-ear scanning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A schematic presentation of a first example embodiment of thedevice according to the invention.

FIG. 2: A schematic presentation of a second example embodiment of thedevice according to the invention (optical correlation).

FIGS. 3A through 3C: Schematic presentations of example embodiments ofpatterns according to the invention.

FIG. 4: A schematic presentation of a first example embodiment of a flatscan tip with large scan length, using a plurality of (dichroic) mirrorsand light sources.

FIG. 5: A schematic presentation of a third example embodiment of a flatscan tip with a large scan length, using a curved mirror.

FIG. 6: A schematic presentation of a fourth example embodiment of aflat scan tip with large scan length, using a diffractive grating.

FIG. 7: A schematic presentation of an example embodiment of amass-balanced focus lens scanner.

FIG. 8: A schematic presentation of an example embodiment of a devicefor simultaneous scanning of a surface shape and color.

FIG. 9: A schematic presentation of an example embodiment of a devicefor scanning the at least a part of the external part of the human earand/or a part of the ear canal a human ear.

FIGS. 10A and 10B: Schematics showing how a scanner embodiment can beused to both scan the outer and inner ear, respectively.

FIG. 11: Schematic of a scanner probe embodiment used to scan a narrowbody cavity, such as a human ear.

FIGS. 12A through 12D: Examples of mirror configurations to be used witha scanner probe.

FIG. 13: A schematic representation of the reference signalvalues/weight values per pixel for a checkerboard pattern in anidealized optical system.

FIGS. 14A through 14E: Illustration of the process of generating a fusedreference signal, visualized as images.

FIG. 15: Top: Example image with projected pattern showing on a humantooth. Bottom: The correlation measure for the series of focus lenspositions at the group of pixels framed in the top part of the figure.

FIG. 16: Example fused correlation measure image of an intraoral scene.

FIG. 17: Example of a handheld intraoral scanner with a pistol-like gripand a removable tip.

It will be understood that the ray traces and lenses depicted in thefigures are for purpose of illustration only, and depict optical pathsgenerally in the discussed systems. The ray traces and lens shapesshould not be understood to limit the scope of the invention in anysense including the magnitude, direction, or focus of light rays orbundles passing through various optical components, not withstanding anyvariations in number, direction, shape, position or size thereof, exceptas expressly indicated in the following detailed description of theexemplary embodiments illustrated in the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

A functional hand held 3D surface scanner should preferably have thefollowing properties:

-   -   1) Telecentricity in the space of the object being scanned,    -   2) possibility to shift the focal plane while maintaining        telecentricity and magnification    -   3) simple focusing scheme that involves tuning of optical        components only in the handle of the device and not in the probe        tip, and    -   4) a total size consistent with a hand held scanning device.

The scanner embodiment illustrated in FIG. 1 is a hand-held scanner withall components inside the housing (head) 100. The scanner head comprisesa tip which can be entered into a cavity, a light source 110, optics 120to collect the light from the light source, pattern generation means130, a beam splitter 140, an image sensor and electronics 180, a lenssystem which transmits and images the light between the pattern, theobject being scanned, and the image sensor (camera) 180. The light fromthe light source 110 travels back and forth through the optical system150. During this passage the optical system images the pattern 130 ontothe object being scanned 200 and further images the object being scannedonto the image sensor 181. The lens system includes a focusing element151 which can be adjusted to shift the focal imaging plane of thepattern on the probed object 200. One way to embody the focusing elementis to physically move a single lens element back and forth along theoptical axis. The device may include polarization optics 160. The devicemay include folding optics 170 which directs the light out of the devicein a direction different to the optical axis of the lens system, e.g. ina direction perpendicular to the optical axis of the lens system. As awhole, the optical system provides an imaging of the pattern onto theobject being probed and from the object being probed to the camera. Oneapplication of the device could be for determining the 3D structure ofteeth in the oral cavity. Another application could be for determiningthe 3D shape of the ear canal and the external part of the ear.

The optical axis in FIG. 1 is the axis defined by a straight linethrough the light source 110, optics 120 and the lenses in the opticalsystem 150. This also corresponds to the longitudinal axis of thescanner illustrated in FIG. 1. The optical path is the path of the lightfrom the light source 110 to the object 220 and back to the camera 180.The optical path may change direction, e.g. by means of beam splitter140 and folding optics 170.

The focus element is adjusted in such a way that the image of thepattern on the scanned object is shifted along the optical axis,preferably in equal steps from one end of the scanning region to theother. When the pattern is varied in time in a periodic fashion for afixed focus position then the in-focus regions on the object willdisplay an spatially varying pattern. The out-of-focus regions willdisplay smaller or no contrast in the light variation. The 3D surfacestructure of the probed object is determined by finding the planecorresponding to an extremum in the correlation measure for each sensorin the camera's sensor array or each group of sensor in the camera'ssensor array when recording the correlation measure for a range ofdifferent focus positions 300. Preferably one would move the focusposition in equal steps from one end of the scanning region to theother.

Pattern Generation

An embodiment of the pattern generation means is shown in FIG. 3a : Atransparent wheel with an opaque mask 133 in the form of spokes pointingradially from the wheel center. In this embodiment the pattern istime-varied by rotating the wheel with a motor 131 connected to thewheel with e.g. a drive shaft 132. The position of the pattern in timemay be registered during rotation. This can be achieved by e.g. using aposition encoder on the rim of the pattern 134 or obtaining the shaftposition directly from motor 131.

FIG. 3b illustrates another embodiment of the pattern generation means:A segmented light source 135, preferably a segmented LED. In thisembodiment the LED surface is imaged onto the object underinvestigation. The individual LED segments 136 are turned on and off ina fashion to provide a known time-varying pattern on the object. Thecontrol electronics 137 of the time varying pattern is connected to thesegmented light source via electrical wires 138. The pattern is thusintegrated into the light source and a separate light source is notnecessary.

FIG. 3c illustrates a static pattern as applied in a spatial correlationembodiment of this invention. The checkerboard pattern shown ispreferred because calculations for this regular pattern are easiest.

Temporal Correlation

FIG. 1 is also an exemplary illustration of the temporal correlationwherein an image of the pattern on and/or in the object is formed on thecamera. Each individual light sensing element in the camera sees avariation in the signal level corresponding to the variation of theillumination pattern on the object. The variation is periodic in theexemplary illustration. The light variation for each individual lightsensing element will have a constant phase offset relative to thepattern position.

The correlation measure may be obtained by recording n images on thecamera during at least one oscillation period. n is an integer numbergreater than one. The registration of the pattern position for eachindividual image combined with the phase offset values for each sensingelement and the recorded images allows for an efficient extraction ofthe correlation measure in each individual sensing element in the camerausing the following formula,

$A_{j} = {\sum\limits_{i = 1}^{n}\;{f_{i,j}I_{i,j}}}$

Here A_(j) is the estimated correlation measure of sensing element j,I_(1,j), . . . I_(n,j) are the n recorded signals from sensing elementj, f_(1,j), . . . f_(n,j) are the n reference signal values obtainedfrom the knowledge of the pattern configuration for each imagerecording. f has two indices i,j. The variation of f with the firstindex is derived from the knowledge of the pattern position during eachimage recording. The variation of f with the second index is derivedfrom the knowledge of the pattern geometry which may be determined priorto the 3D scanning.

The focus position corresponding to the pattern being in focus on theobject for a single sensor in the camera will be given by an extremum inthe recorded correlation measure of that sensor when the focus positionis varied over a range of values, preferably in equal steps from one endof the scanning region to the other.

Spatial Correlation

In an example of the spatial correlation scheme, one image of the objectwith projected checkerboard pattern is recorded with as high resolutionas allowed by the image sensor. The scheme in the spatial correlation inis then to analyze groups of pixels in the recorded image and extractthe correlation measure in the pattern. An extremum in the obtainedcorrelation measures indicates the in-focus position. For simplicity,one can use a checkerboard pattern with a period corresponding to n=N×Npixels on the sensor and then analyze the correlation measure within oneperiod of the pattern (in the general case the pattern need not bequadratic N×N). In the best case, it will be possible to align thepattern so that the checkerboard edges coincide with the pixel edges butthe scanning principle does not rely upon this. FIG. 16 shows this forthe case n=4×4=16. For a sensor with W×H=1024×512 pixels, this wouldcorrespond to obtaining 256×128 correlation measure points from oneimage. Extraction of the correlation measure A_(j) within an N×N groupof pixels with label j is given by

$A_{j} = {\sum\limits_{i = 1}^{n}\;{f_{i,j}I_{i,j}}}$where f_(j)=(f_(1,j), . . . f_(n,j)) is the reference signal vectorobtained from knowledge of the pattern configuration, andI_(j)=(I_(1,j), . . . I_(n,j)) is input signal vector.

To suppress any DC part in the light we prefer that for all j that

$0 = {\sum\limits_{i = 1}^{n}\; f_{i,j}}$

For the situation depicted in FIG. 16 for instance, f_(i,j)=−1 for thepixels corresponding to the dark parts of the pattern, and f_(i,j)=+1otherwise. If the pattern edge was not aligned with the edges of thepixels, or if the optical system was not perfect (and thus in allpractical applications), then f_(i,j) would assume values between −1 and+1 for some i. A detailed description of how to determine the referencefunction is given later.

Optical Correlation

An example of the optical correlation shown in FIG. 2. In thisembodiment an image is formed on the camera 180 which is a superpositionof the pattern 130 with the probed object 200. In this embodiment thepattern is of a transmissive nature where light is transmitted throughthe pattern and the image of the pattern is projected onto the objectand back again. In particular this involves retransmission of the lightthrough the pattern in the opposite direction. An image of the patternonto the camera is then formed with the aid of a beam splitter 140. Theresult of this arrangement is an image being formed on the camera whichis a superposition of the pattern itself and the object being probed. Adifferent way of expressing this is that the image on the camera issubstantially a multiplication of an image of the pattern projected ontothe object with the pattern itself.

The variation is periodic in the exemplary illustration. The correlationmeasure between the light variation on the object and the pattern for agiven focus distance may be obtained by time integrating the camerasignal over a large number of oscillation periods so that exactsynchronization of pattern oscillation time and camera integration timeis not important. The focus position corresponding to the pattern beingin focus on the object for a single sensor in the camera will be givenby the maximum recorded signal value of that sensor when the focusposition is varied over a range of values, preferably in equal stepsfrom one end of the scanning region to the other.

Finding the Predetermined Reference Function

In the following, the process for computing the reference signal f isdescribed for a spatial correlation embodiment of this invention, anddepicted in a stylized way in FIGS. 14A-14E.

The process starts by recording a series of images of the checkerboardpattern as projected, e.g., on a flat surface, preferably orientedorthogonally to the optical axis of the scanner. The images are taken atdifferent positions of the focusing element, in effect covering theentire travel range of said focus element. Preferably, the images aretaken at equidistant locations.

As the focus plane generally is not a geometrical plane, differentregions of the flat surface will be in focus in different images.Examples of three such images are shown in FIGS. 14A-14C, where 1700 isan in-focus region. Note that in this stylized figure, transitionsbetween regions in and out of focus, respectively, are exaggerated inorder to demonstrate the principle more clearly. Also, in general therewill be many more images than just the three used in this simpleexample.

In-focus regions within an image are found as those of maximum intensityvariance (indicating maximum contrast) over the entire said series ofimages. The region to compute variance over need not be the same as thepixel group dimension used in spatial correlation, but should be largeenough to contain the both dark and light regions of the pattern, and itmust be the same for all images in the series.

Finally, a “fused image” (FIG. 14D) is generated by combining all thein-focus regions of the series (14A-14C). Note that in realapplications, the fused image will generally not be a perfectcheckerboard of black and white, but rather include intermediate grayvalues as caused by an imperfect optical system and a checkerboard thatis not perfectly aligned with the camera sensors. An example of part ofa real fused image is shown in FIG. 14E.

The pixel intensities within this image can be interpreted as a “weightimage” with same dimensions as the original image of the pattern. Inother words, the pixel values can be interpreted as the reference signaland the reference vector/set of weight values f_(j)=(f_(1,j), . . .f_(n,j)) for the n pixels in the pixel group with index j can be foundfrom the pixel values.

For convenience in the implementation of the calculations, especiallywhen carried out on an FPGA, the fused image can be sub-divided intopixel groups. The DC part of the signal can then be removed bysubtracting the within-group intensity mean from each pixel intensityvalue. Furthermore, one can then normalize by dividing by thewithin-group standard deviation. The thus processed weight values are analternative description of the reference signal.

Because of the periodic nature of the “fused image” and thus the “weightimage”, the latter can be compressed efficiently, thus minimizing memoryrequirements in the electronics that can implement the algorithmdescribed here. For example, the PNG algorithm can be used forcompression.

The “Correlation Image”

An “correlation” image is generated based on the “fused image” and theset of images recorded with the camera during a scan. For spatialcorrelation based on an N×N checkerboard pattern, recall thatwithin-group correlation measure isA _(j)=Σ_(i=1) ^(N×N) f _(i,j) I _(i,j),where f_(j)=(f_(1,j), . . . f_(n,j)) are values from the fused image,and I_(j)=(I_(1,j), . . . I_(n,j)) are values from a recorded image onthe camera. The pixel groupings used in any DC removal and possiblynormalization that yielded the fused image are the same as in the abovecalculation. For each image recorded by the scanner during a sweep ofthe focusing element, there will thus be an array of (H/N)×(W/N) valuesof A. This array can be visualized as an image.

FIG. 15 (top section) shows one example correlation measure image, hereof part of a human tooth and its edge. A pixel group of 6×6 pixels ismarked by a square 1801. For this example pixel group, the series ofcorrelation measures A over all images within a sweep of the focusingelement is shown in the chart in the bottom section of FIG. 15 (crosshairs). The x-axis on the chart is the position of the focusing element,while the y-axis shows the magnitude of A. Running a simple Gaussianfilter over the raw series results in a smoothed series (solid line). Inthe figure the focus element is in the position that gives optimal focusfor the example group of pixels. This fact is both subjectively visiblein the picture, but also determined quantitatively as the maximum of theseries of A. The vertical line 1802 in the bottom section of FIG. 15indicates the location of the global extremum and thus the in-focusposition. Note that in this example, the location of the maxima in thesmoothed and the raw series, respectively, are visuallyindistinguishable. In principle, however, it is possible and alsoadvantageous to find the maximum location from the smoothed series, asthat can be between two lens positions and thus provide higher accuracy.

The array of values of A can be computed for every image recorded in asweep of the focus element. Combining the global extrema (over allimages) of A in all pixel groups in the same manner the fused image wascombined, one can obtain a pseudo-image of dimension (H/N)×(W/N). Thiswe call the “fused correlation image”. An example of a fused correlationimage of some teeth and gingiva is shown in FIG. 19. As can be seen, itis useful for visualization purposes.

Increasing Field of View

For the scan head to be entered into a small space such as the oralcavity of a patient it is necessary that the tip of the scan head issufficiently small. At the same time the light out of the scan head needto leave the scan head in a direction different from the optical axis,e.g. at a direction perpendicular to the optical axis. In one embodimentof the invention a 45 degree mirror is used as folding optics 170 directthe light path onto the object. In this embodiment the height of thescan tip need to be at least as large as the scan length.

Another embodiment of the invention is shown in FIG. 4. This embodimentof the invention allows for a scan tip with a smaller height (denoted bin the figure) than the scan length (denoted a in the figure). The lightfrom two sources 110 and 111 emitting light of differentcolors/wavelengths is merged together using a suitably coated plate(e.g. a dichroic filter) 112 that transmit the light from 110 andreflects the light from 111. At the scan tip a suitably coated plate(e.g. a dichroic filter) 171 reflects the light from one source onto theobject and transmits the light from the other source to a mirror at theend of the scan tip 172. During scanning the focus position is movedsuch that the light from 110 is used to project an image of the patternto a position below 171 while 111 is switched off. The 3D surface of theobject in the region below 171 is recorded. Then 110 is switched off and111 is switched on and the focus position is moved such that the lightfrom 111 is used to project an image of the pattern to a position below172. The 3D surface of the object in the region below 172 is recorded.The region covered with the light from 110 and 111 respectively maypartially overlap.

Another embodiment of the invention that allows for a scan tip with asmaller height (denoted b in the figure) than the scan length (denoted ain the figure) is shown in FIG. 5. In this embodiment the fold optics170 comprises a curved fold mirror 173 that may be supplemented with oneor two lens elements 175 and 176 with surfaces that may be aspherical toprovide corrected optical imaging.

Another embodiment of the invention that allows for a scan tip with asmaller height (denoted b in the figure) than the scan length (denoted ain the figure) is shown in FIG. 6. In this embodiment the fold optics170 comprises a grating 177 that provides anamorphic magnification sothat the image of the pattern on the object being scanned is stretched.The grating may be blazed. The light source 110 needs to bemonochromatic or semi-monochromatic in this embodiment.

Achieving High Spatial Contrast of Pattern Projected onto DifficultObjects

High spatial contrast of the in-focus pattern image on the object isnecessary to obtain a high correlation measure signal based on thecamera pictures. This in turn is necessary to obtain a good estimationof the focus position corresponding to the position of an extremum ofthe correlation measure. This necessary condition for successfulscanning is easily achieved in objects with a diffuse surface andnegligible light penetration. For some objects, however, it is difficultto achieve high spatial contrast, or more generally variation.

A difficult kind of object, for instance, is an object displayingmultiple scattering with a light diffusion length large compared to thesmallest feature size of the spatial pattern imaged onto the object. Ahuman tooth is an example of such an object. The human ear and ear canalare other examples. Improved spatial variation in such objects can beachieved by preferential imaging of the specular surface reflection fromthe object on the camera. An embodiment of the invention appliespolarization engineering shown in FIG. 1. In this embodiment the beamsplitter 140 is a polarizing beam splitter that transmits respectivelyreflects two orthogonal polarization states, e.g. S- and P-polarizationstates. The light transmitted through the lens system 150 is thus of aspecific polarization state. Before leaving the device the polarizationstate is changed with a retardation plate 160. A preferred type ofretardation plate is a quarter wave retardation plate. A linearlypolarized light wave is transformed into a circularly polarized lightwave upon passage of a quarter wave plate with an orientation 45 degreesof its fast axis to the linear polarization direction. A specularreflection from the object has the property that it flips the helicityof a circularly polarized light wave. Upon passage of the quarter waveretardation plate by the specularly reflected light the polarizationstate becomes orthogonal to the state incident on the object. Forinstance an S-polarization state propagating in the downstream directiontoward the object will be returned as a P-polarization state. Thisimplies that the specularly reflected light wave will be directedtowards the image sensor 181 in the beam splitter 140. Light that entersinto the object and is reflected by one or more scattering eventsbecomes depolarized and one half of this light will be directed towardsthe image sensor 181 by the beam splitter 140.

Another kind of difficult object is an object with a shiny ormetallic-looking surface. This is particularly true for a polishedobject or an object with a very smooth surface. A piece of jewelry is anexample of such an object. Even very smooth and shiny objects, however,do display an amount of diffuse reflection. Improved spatial contrast insuch objects can be achieved by preferential imaging of the diffusesurface reflection from the object on the camera. In this embodiment thebeam splitter 140 is a polarizing beam splitter that transmitsrespectively reflects two orthogonal polarization states, e.g. S- andP-polarization states. The light transmitted through the lens system 150is thus of a specific polarization state. A diffuse reflection from theobject has the property that it loses its polarization. This impliesthat half of the diffusely reflected light wave will be directed towardsthe image sensor 181 in the beam splitter 140. Light that enters intothe object and is reflected by specular polarization preserves itspolarization state and thus none of it will be directed towards theimage sensor 181 by the beam splitter 140.

Reducing Shaking Caused by Focus Element

During scanning the focus position is changed over a range of values,preferably provided by a focusing element 151 in the optical system 150.FIG. 7 illustrates an example of how to reduce shaking caused by theoscillating focus element. The focusing element is a lens element 152that is mounted on a translation stage 153 and translated back and forthalong the optical axis of said optical system with a mechanicalmechanism 154 that includes a motor 155. During scanning the center ofmass of the handheld device is shifted due to the physical movement ofthe lens element and holder. This results in an undesirable shaking ofthe handheld device during scanning. The situation is aggravated if thescan is fast, e.g. a scan time of less than one second. In oneimplementation of the invention the shifting of the center of mass iseliminated by moving a counter-weight 156 in a direction opposite to thelens element in such a way that the center of mass of the handhelddevice remains fixed. In the preferred implementation the focus lens andthe counter-weight are mechanically connected and their oppositemovement is driven by the same motor.

Color Measurement

An embodiment of a color 3D scanner is shown in FIG. 8. Three lightsources 110, 111, and 113 emit red, green, and blue light. The lightsources are may be LEDs or lasers. The light is merged together tooverlap or essentially overlap. This may be achieved by means of twoappropriately coated plates 112 and 114. Plate 112 transmits the lightfrom 110 and reflects the light from 111. Plate 114 transmits the lightfrom 110 and 111 and reflects the light from 113. The color measurementis performed as follows: For a given focus position the amplitude of thetime-varying pattern projected onto the probed object is determined foreach sensor element in the sensor 181 by one of the above mentionedmethods for each of the light sources individually. In the preferredembodiment only one light source is switched on at the time, and thelight sources are switched on after turn. In this embodiment the opticalsystem 150 may be achromatic. After determining the amplitude for eachlight source the focus position is shifted to the next position and theprocess is repeated. The color expressed as e.g. an RGB color coordinateof each surface element can be reconstructed by appropriate weighting ofthe amplitude signal for each color corresponding the maximum amplitude.

One specific embodiment of the invention only registers the amplitudefor all colors at an interval of P focus positions; while one color isselected for determination of the amplitude at all focus positions. P isa number which could be e.g. 3, 5, or 10. This results in a colorresolution which is less than the resolution of the surface topology.Color of each surface element of the probed object is determined byinterpolation between the focus positions where full color informationis obtained. This is in analogy to the Bayer color scheme used in manycolor digital cameras. In this scheme the color resolution is also lessthan the spatial resolution and color information need to beinterpolated.

A simpler embodiment of the 3D color scanner does not register fullcolor information and employs only two light sources with differentcolors. An example of this is a dental scanner that uses red and bluelight to distinguish hard (tooth) tissue from soft (gum) tissue.

Ear Scanner Embodiment

FIGS. 9-12 schematically illustrate an embodiment of a time-varyingstructured light illumination-based scanner for direct scanning of humanears by scanning both the exterior (outer) and interior (inner) part ofa human ear by use of a common scanner exterior handle and a detachableprobe. This embodiment is advantageous in that it allows fornon-intrusive scanning using a probe designed to be inserted into smallcavities, such as a human ear. This is done in part by positioning thebulky and essential parts of the scanner, such as the scanner camera,light source, electronics and focusing optics outside the closelyconfined part of the ear canal.

The ability to scan the outer and inner part of human ears and make avirtual or real model of the ear is essential in the design of moderncustom-fitted hearing aid (e.g. ear shell or mold). Today, scanning ofears is performed in a two-step process where a silicone impression ofthe ear is taken first and the impression is subsequently scanned usingan external scanner in a second step. The process of making theimpression suffers from several drawbacks which will shortly bedescribed in the following. One major drawback comes from frequent poorquality impressions taken by qualified clinic professionals due to thepreparation and techniques required. Inaccuracies may arise because theimpression material is known to expand during hardening and thatdeformation and creation of fractures in the impression are oftencreated when the impression is removed from the ear. Another drawback isrelated to health risks involved with taking the impression due toirritation and allergic responses, damage to the tympanic membrane andinfections. Finally, the impression process is an uncomfortableexperience for many patients, especially for young children, who oftenrequire impressions taken at regular intervals (e.g. every four months)to accommodate the changing dimensions of the ear canal. In short, thesedrawbacks can be overcome if it is possible to scan the outer and innerear in a non-intrusive way and obtain a registration between the innerand outer ear surfaces.

The following is not restricted to ear scanning but can be used to scanany small bodily cavity. FIG. 9 is a schematic of an embodiment of sucha scanner. The scanner consists of two main parts—a scanner exterior1001 and a scanner probe 1002. The scanner exterior may be used withoutthe probe to obtain a larger field-of-view needed e.g. to scan theexterior part of the ear 1102, or the first part of the ear canal up tothe first bend. The large field-of-view of the scanner exterior isimportant to obtain good registration between individual sub-scans andhigh global accuracy. By attaching a scanner probe 1202 to the scannerexterior 1201, the combined scanner allows for scanning of small andbent cavity surfaces, such as the interior part of an ear 1203. In thisway and using the same system, the combined scanner exterior and probeare able to both scan larger external areas along with smaller internalareas. In FIG. 9 the exterior part of the scanner embodiment 1001consists of a diverging light source 1003 (laser, LED, Tungsten oranother type) which is collimated using collimation optics 1004. Thecollimated light is used to illuminate a transparent object 1005 (e.g.glass) with an opaque pattern, e.g. fringes on it. The pattern issubsequently imaged onto the object to be scanned using a suitableoptical system. The pattern is observed using a similar optical systemand a camera 1006, where the latter is positioned outside the cavity.The 3D information is obtained from the 2D images by observing the lightoscillation created by the movement of the pattern across the scanobject as contained in the individual pixel amplitude.

To facilitate movement of the pattern, the fringe pattern 1005 isrotating in one embodiment. In another embodiment, the fringe pattern ispositioned on a translating plate that moves in a plane perpendicular tothe optical axis with a certain oscillation frequency. The light to andfrom the scan object is projected through a beam splitter arrangement1007, which consists of a prism cube in one embodiment and in anotherembodiment consists of an angled plate or membrane. The beam splitterserves to transmit the source light further down the system, while atthe same time guide the reflected light from the scan object back to thecamera, which is positioned on an axis perpendicular to the axis of thelight source and beam splitter.

To move the focus plane the scanner exterior includes focusing optics,which in one embodiment consists of a single movable lens 1008. Thepurpose of the focusing optics is to facilitate movement of the plane offocus for the whole imaging system in the required scanning range andalong the optical axis. In one embodiment, the focusing optics of thescanner exterior 1101 includes an objective that can focus the lightdirectly, without any use of additional optics, as shown in FIG. 10A. Inanother embodiment, the scanner exterior is supplied with a wide-angleobjective designed with a large field-of-view, e.g. sufficiently largefor scanning the exterior part of a human ear 1102.

The optical part of the scanner probe consists of an endoscopic opticalrelay system 1009 followed by a probe objective 1010, both of which areof sufficiently small diameter to fit into the canal of a human ear.These optical systems may consist of both a plurality of optical fibersand lenses and serve to transport and focus the light from the scannerexterior onto the scan object 1014 (e.g. the interior surface of anear), as well as to collimate and transport the reflected light from thescan object back to the scanner exterior. In one embodiment, the probeobjective provides telecentric projection of the fringe pattern onto thescan object. Telecentric projection can significantly ease the datamapping of acquired 2D images to 3D images. In another embodiment, thechief rays (center ray of each ray bundle) from the probe objective arediverging (non-telecentric) to provide the camera with an angle-of-viewlarger than zero, as shown in FIG. 9.

The position of the focus plane is controlled by the focusing optics1008 and can be moved in a range large enough to at least coincide withthe scan surface 1014. A single sub-scan is obtained by collecting anumber of 2D images at different positions of the focus plane and atdifferent positions of the fringe pattern, as previously described. Asthe focus plane coincides with the scan surface at a single pixelposition, the fringe pattern will be projected onto the surface pointin-focus and with high contrast, thereby giving rise to a largevariation, or amplitude, of the pixel value over time. For each pixel itis thus possible to identify individual settings of the focusing opticsfor which each pixel will be in-focus. By using knowledge of the opticalsystem, it is possible to transform the contrast information vs.position of the focus plane into 3D surface information, on anindividual pixel basis.

In one embodiment, a mirror arrangement 1011, consisting of a singlereflective mirror, or prism, or an arrangement of mirrors, are locatedafter the probe objective 1010. This arrangement serves to reflect therays to a viewing direction different from that of the of the probeaxis. Different example mirror arrangements are found in FIGS. 12A-12D.In one particular embodiment, the angle between the mirror normal andthe optical axis is approximately 45 degrees, thus providing a 90 degreeview with respect to the probe axis—an arrangement ideal for lookinground corners. A transparent window 1012 is positioned adjacent to themirror and as part of the probe casing/shell, to allow the light to passbetween the probe and the scan object, while keeping the optics cleanfrom outside dirt particles.

To reduce the probe movement required by a scanner operator, the mirrorarrangement may be rotated using a motor 1013. In one embodiment, themirror arrangement rotates with constant velocity. By full rotation of asingle mirror, it is in this way possible to scan with 360 degreecoverage around the probe axis without physically moving the probe. Inthis case, the probe window 1012 is required to surround/go all aroundthe probe to enable viewing in every angle. In another embodiment, themirror rotates with a certain rotation oscillation frequency. In yetanother embodiment, the mirror arrangement tilt with respect to theprobe axis is varied with a certain oscillation frequency.

A particular embodiment uses a double mirror instead of a single mirror(FIGS. 12B and 12D). In a special case, the normal of the two mirrorsare angled approx. 90 degrees with respect to each other. The use of adouble mirror helps registration of the individual sub-scans, sinceinformation of two opposite surfaces in this way is obtained at the sametime. Another benefit of using a double mirror is that only 180 degreesof mirror rotation is required to scan a full 360 degrees. A scannersolution employing double mirrors may therefore provide 360 degreescoverage in less time than single mirror configurations.

“Pistol-like” Grip

FIG. 17 shows an embodiment of the scanner with a pistol-like grip 2001.This form is particularly ergonomic. The scanner in FIG. 17 is designedfor intra-oral scanning of teeth. The tip 2002. can be removed from themain body of the scanner and can be autoclaved. Furthermore, the tip canhave two positions relative to the main body of the scanner, namelylooking down (as in FIG. 17) and looking up. Therefore, scanning theupper and the lower mouth of a patient is equally comfortable for theoperator. Note that the scanner shown in FIG. 17 is an early prototypewith several cables attached for testing purposes only.

Although some embodiments have been described and shown in detail, theinvention is not restricted to them, but may also be embodied in otherways within the scope of the subject matter defined in the followingclaims. In particular, it is to be understood that other embodiments maybe utilised and structural and functional modifications may be madewithout departing from the scope of the present invention.

In device claims enumerating several means, several of these means canbe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims ordescribed in different embodiments does not indicate that a combinationof these measures cannot be used to advantage.

It should be emphasized that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof.

The features of the method described above and in the following may beimplemented in software and carried out on a data processing system orother processing means caused by the execution of computer-executableinstructions. The instructions may be program code means loaded in amemory, such as a RAM, from a storage medium or from another computervia a computer network. Alternatively, the described features may beimplemented by hardwired circuitry instead of software or in combinationwith software.

The invention claimed is:
 1. A scanner for determining the 3D geometryof at least a part of the surface of an object, said scanner comprising:at least one camera accommodating an array of sensor elements, a lightsource for generating a probe light, an optical system for transmittingthe probe light towards the object along an optical path therebyilluminating at least a part of the object in one or more configurationsand for transmitting at least a part of the light returned from theobject to the camera, and a processor configured to: determine an infocus position for each of a plurality of image pixels, or each of aplurality of groups of image pixels, at least partly by comparing areference signal representing the probe light with a signal representingthe light returned from the object to the camera, and using the in focusposition for each of a plurality of image pixels, or each of a pluralityof groups of image pixels, to generate 3D surface information.
 2. Thescanner according to claim 1, wherein the probe light incorporates aspatial pattern.
 3. The scanner according to claim 1, wherein thehardware processor is further configured to generate, for each of aplurality of image pixels or each of a plurality of groups of imagepixels, a correlation measure, for each of a number of focus planepositions along the optical path, in a range of focus plane positions,and identify an extremum of the correlation measures generated for eachof the plurality of image pixels or each of the plurality of groups ofimage pixels, to determine an in-focus plane position for each of theplurality of image pixels or each of the plurality of groups of imagepixels.
 4. The scanner according to claim 1, wherein the in-focus planeposition for said pixel or group of pixels is determined as an at leastlocal extremum position of an optionally smoothed series of correlationmeasures calculated for the range of said focus plane positions.
 5. Thescanner according to claim 1, wherein the object is an ear canal or atooth.
 6. The scanner according to claim 1, further comprising at leastone beam splitter located in the optical path.
 7. The scanner accordingto claim 1, wherein the focus plane position is periodically varied witha predefined frequency.
 8. The scanner according to claim 1, furthercomprising means for fixing and/or maintaining a center of mass of thefocus element adjustment means.
 9. The scanner according to claim 1,further comprising a counter-weight to substantially counter-balancemovement of the focus element.
 10. The scanner according to claim 2,wherein the spatial pattern is a static line pattern or a staticcheckerboard pattern.
 11. The scanner according to claim 1, wherein thelight source is a segmented light source.
 12. The scanner according toclaim 1, wherein the sensor element array is divided into groups ofsensor elements.
 13. The scanner according to claim 2, wherein the imageof the spatial pattern is a line pattern or a checkerboard pattern, andis aligned with the rows and/or the columns of the array of sensorelements.
 14. The scanner according to claim 1, further comprising aquarter wave retardation plate and a linearly polarizing element locatedin the optical path.
 15. The scanner according to claim 1, furthercomprising at least one optical device which is partially lighttransmitting and partially light reflecting, said optical device locatedalong an optical axis of the input signal.
 16. The scanner according toclaim 1, further comprising a reflective element for directing the probelight in a different direction and means for rotating the reflectiveelement.
 17. The scanner according to claim 1, wherein the processor isfurther configured to generate, for each of a plurality of image pixelsor each of a plurality of groups of image pixels, a correlation measure,for each of a number of focus plane positions along the optical path, ina range of focus plane positions, at least partly by comparing areference signal representing the probe light with an input signalrepresenting the light returned from the object to the camera.
 18. Thescanner according to claim 16, wherein the processor is furtherconfigured to identify an extremum of the correlation measures generatedfor each of the plurality of image pixels or each of the plurality ofgroups of image pixels to determine an in-focus plane position for eachof the plurality of image pixels or each of the plurality of groups ofimage pixels.